U.S. patent application number 11/664216 was filed with the patent office on 2008-02-28 for optical element, polarization plane light source using the optical element, and display device using the polarization plane light source.
Invention is credited to Kazutaka Hara, Minoru Miyatake.
Application Number | 20080049317 11/664216 |
Document ID | / |
Family ID | 36118983 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080049317 |
Kind Code |
A1 |
Hara; Kazutaka ; et
al. |
February 28, 2008 |
Optical Element, Polarization Plane Light Source Using the Optical
Element, and Display Device Using the Polarization Plane Light
Source
Abstract
The present invention provides an optical element including: a
translucent resin; minute regions dispersedly distributed in the
translucent resin and having a birefringence different from the
translucent resin; and at lest one kind of luminous body dispersed
in the translucent resin and/or the minute regions and having a
particle size smaller than the emission wavelength thereof, the
optical element having a plate-like shape; a
polarized-light-emitting planar light source including the optical
element and the excitation light source; and a display device
including the polarized-light-emitting planar light source.
Inventors: |
Hara; Kazutaka; (Osaka,
JP) ; Miyatake; Minoru; (Osaka, JP) |
Correspondence
Address: |
SUGHRUE-265550
2100 PENNSYLVANIA AVE. NW
WASHINGTON
DC
20037-3213
US
|
Family ID: |
36118983 |
Appl. No.: |
11/664216 |
Filed: |
September 28, 2005 |
PCT Filed: |
September 28, 2005 |
PCT NO: |
PCT/JP05/17886 |
371 Date: |
March 29, 2007 |
Current U.S.
Class: |
359/487.02 ;
359/487.06; 359/493.01 |
Current CPC
Class: |
G02B 5/0242 20130101;
G02B 6/0065 20130101; G02F 1/13362 20130101; G02B 6/0041 20130101;
G02B 5/0278 20130101; G02B 5/3008 20130101; G02B 5/0284 20130101;
G02B 5/0247 20130101; G02B 6/0056 20130101 |
Class at
Publication: |
359/483 |
International
Class: |
G02B 5/30 20060101
G02B005/30 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2004 |
JP |
2004-288122 |
Apr 20, 2005 |
JP |
2005-122721 |
Claims
1. An optical element comprising: a translucent resin; minute
regions dispersedly distributed in the translucent resin and having
a birefringence different from the translucent resin; and at lest
one kind of luminous body dispersed in the translucent resin and/or
the minute regions and having a particle size smaller than the
emission wavelength thereof, the optical element having a
plate-like shape.
2. The optical element according to claim 1, wherein the luminous
body is an inorganic pigment.
3. The optical element according to claim 1, wherein the luminous
body is a fluorescent pigment absorbing ultraviolet light or
visible light and emitting visible light.
4. The optical element according to claim 1, wherein the luminous
body is a phosphorescent pigment absorbing ultraviolet light or
visible light and emitting visible phosphorescence.
5. The optical element according to claim 1, wherein the particle
size of the luminous body is not more than one fifth of the
emission wavelength of the luminous body.
6. The optical element according to claim 1, wherein the diameter
of aggregate formed by aggregating the luminous body is smaller
than the emission wavelength of the luminous body.
7. The optical element according to claim 1, wherein the
translucent resin and the minute regions both are made of materials
substantially not absorbing ultraviolet light.
8. The optical element according to claim 1, wherein the minute
regions are at least one selected from the group consisting of: a
liquid crystalline material; a glass state material formed by
cooling and fixing a liquid crystal phase; and a material formed by
crosslinking and fixing a liquid crystal phase of a polymerizable
liquid crystal with an energy ray.
9. The optical element according to claim 1, wherein the minute
regions are liquid crystal polymer having a glass transition
temperature of 50.degree. C. or higher and exhibiting a nematic
liquid crystal phase at a temperature lower than the glass
transition temperature of the translucent resin.
10. The optical element according to claim 1, which satisfies the
following relations: 0.03.ltoreq..DELTA.n1.ltoreq.0.5
0.ltoreq..DELTA.n2.ltoreq.0.03 0.ltoreq..DELTA.n3.ltoreq.0.03
provided that: .DELTA.n1 is the difference of refractive indices
between the translucent resin and the minute regions in an axial
direction of the minute regions along which the difference between
the translucent resin and the minute regions indicates the maximum
value; and .DELTA.n2 and .DELTA.n3 are differences of refractive
indices between the translucent resin and the minute regions in
axial directions each orthogonal to the axial direction along which
the maximum value is indicated.
11. A polarized-light-emitting planar light source comprising: the
optical element according to claim 1; and an excitation light
source emitting a light of a wavelength being capable of exciting
the luminous body dispersed in the optical element.
12. The polarized-light-emitting planar light source according to
claim 11, wherein the translucent resin and the minute regions both
are made of materials substantially not absorbing ultraviolet
light, and the light of a wavelength being capable of exciting the
luminous body dispersed in the optical element is ultraviolet
light.
13. The polarized-light-emitting planar light source according to
claim 11, which further comprising a light guide member made of a
translucent material.
14. The polarized-light-emitting planar light source according to
claim 11, wherein the excitation light source is an inorganic or
organic electroluminescent element or a mercury-free fluorescent
tube.
15. A display device comprising the polarized-light-emitting planar
light source according to claim 11.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an optical element and a
polarized-light-emitting planar light source using the same as well
as a display device using the same. Particularly, the present
invention relates to an optical element that is capable of allowing
light, which results from excitation by incident light, to be
emitted through at least one of front and rear sides thereof in the
form of linearly polarized light having a predetermined plane of
vibration, as well as a polarized-light-emitting planar light
source using the same and a display device using the same.
BACKGROUND ART
[0002] Heretofore, as a side-light type light-guiding plate used in
a so-called backlight of a liquid crystal display, there is known
one wherein a light emitting means made up of reflective dots
containing high-reflectance pigments such as titanium oxide or
barium sulfate is provided on a translucent resin plate and the
light guide emits light from one of the front and rear sides of the
resin plate through the light emitting means by scattering light,
which is transmitted in the resin plate upon total internal
reflection.
[0003] However, since the light emitted from the light-guiding
plate having the above arrangement is natural light that exhibits
almost no polarization characteristics, it is necessary to convert
the emitted light into linearly polarized light via a polarizing
plate when it is used for a liquid crystal display. Therefore, the
conversion causes absorption loss of light by the polarizing plate
and hence there is a problem that the utilization rate of light
cannot exceed 50%.
[0004] In order to address such a problem, various backlights
wherein increase in utilization rate of light is attempted by
employing a polarization splitting means that produces linearly
polarized light utilizing a so-called Brewster's angle or by
employing a polarization converting means using a retardation plate
have been proposed (see e.g., Patent Documents 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11, 12, and 13).
[0005] However, since those conventional backlights still cannot
attain a sufficient degree of polarization and are hard to control
the polarization direction, there is a problem that they are of
little practical use.
[0006] Thus, in order to solve the above problem, the inventors of
the invention have already developed an optical element that is
capable of allowing light, which results from excitation by
incident light, to be emitted through at least one of the front and
rear sides of the optical element in the form of linearly polarized
light having a predetermined plane of vibration and also is capable
of optionally controlling the polarization direction (plane of
vibration) (Patent Document 14).
[0007] However, in the point of time when the optical element
described in Patent Document 14 was developed, although a highly
qualitative inference that it is preferable to reduce the size of
the luminous body dispersed in the translucent resin and/or minute
regions as far as possible in a case where non-dissolving luminous
body is used (see paragraph 0026 of the specification of Patent
Document 14), any specific quantitative study on the size to be
reduced was not conducted at all.
[0008] For example, in Patent Document 14, an example using a
powder of tris(8-quinolinolato)aluminum (generally referred to as
Alq3) is disclosed but Alq3 used in the example, which is
commercially available, has a particle size of several tens .mu.m.
In a case where an optical element is made using a luminous body
having such a degree of particle size, it is found that lights,
which results from excitation light entering the optical element
and is emitted to outside of the optical element, has not
necessarily a sufficient degree of polarization in some cases.
Moreover, in a case where a luminous body having a particle size
larger than a predetermined one is used, there are problems that
defective appearance of the optical element may occur or a case
where preparation of an optical element is difficult to prepare may
occur. Furthermore, when it is attempted to increase a mixing ratio
of the luminous body to be dispersed in the optical element, it is
impossible to disperse the luminous body in a large amount, so that
there is a problem that luminance cannot be effectively
enhanced.
[0009] Patent Document 1: JP-A-6-18873
[0010] Patent Document 2: JP-A-6-160840
[0011] Patent Document 3: JP-A-6-265892
[0012] Patent Document 4: JP-A-7-72475
[0013] Patent Document 5: JP-A-7-261122
[0014] Patent Document 6: JP-A-7-270792
[0015] Patent Document 7: JP-A-9-54556
[0016] Patent Document 8: JP-A-9-105933
[0017] Patent Document 9: JP-A-9-138406
[0018] Patent Document 10: JP-A-9-152604
[0019] Patent Document 10: JP-A-9-293406
[0020] Patent Document 12: JP-A-9-326205
[0021] Patent Document 13: JP-A-10-78581
[0022] Patent Document 14; JP-A-2004-205953
DISCLOSURE OF THE INVENTION
Problems to be Resolved by the Invention
[0023] The present invention is contrived for the purpose of
solving such problems in the conventional technology and an object
of the invention is to provide an optical element that is capable
of allowing light, which results from excitation by incident light
to be emitted through at least one of the front and rear sides of
the optical element in the form of linearly polarized light having
a sufficient degree of polarization and that is easily prepared
without occurrence of defective appearance and is capable of easily
enhancing the luminance of emitted light as well as a
polarized-light-emitting planar light source using the optical
element and a display device using the same.
Means of Solving the Problems
[0024] As a result of the extensive studies for solving the above
problems, the present inventors have found that reduction of the
particle size of the luminous body dispersed in the translucent
resin and/or the minute regions to a particle size smaller than the
emission wavelength thereof affords an optical element that is
capable of allowing light, which results from excitation by
incident light, to be emitted through at least one of the front and
rear sides of the optical element in the form of linearly polarized
light having a sufficient degree of polarization and that is easily
prepared without occurrence of defective appearance and is capable
of easily enhancing the luminance of emitted light. Thus, they have
accomplished the invention.
[0025] Namely, the invention provides an optical element
comprising: a translucent resin; minute regions dispersedly
distributed in the translucent resin and having a birefringence
different from the translucent resin; and at lest one kind of
luminous body dispersed in the translucent resin and/or the minute
regions and having a particle size smaller than the emission
wavelength thereof, the optical element having a plate-like
shape.
[0026] According to the invention, the thus arranged optical
element omits the necessity to provide a special light emitting
means made of reflective dots or the like on a translucent resin as
before, while being capable of allowing light, which results from
excitation by incident light in the optical element (the luminous
body), to be emitted to the outside in the form of linearly
polarized light having a predetermined plane of vibration. Also,
the optical element of the invention can optionally set the
polarization direction (plane of vibration) of linearly polarized
light according to the installation angle of the optical element
(according to which direction is designated as a .DELTA.n1
direction hereinafter described).
[0027] More specifically, most of the light, which results from
excitation by excitation light entering the optical element through
a lateral side or front or rear side thereof, is totally reflected
at an air interface according to the refractive index difference
between the optical element and air; and transmitted within the
optical element. Of the transmitted light, a linearly polarized
light component having a plane of vibration parallel to the axial
direction (the .DELTA.n1 direction) of the minute regions, along
which direction a maximum difference (.DELTA.n1) in refractive
index between the minute regions and the transparent resin occurs,
is selectively and strongly scattered. Of the scattered light,
light scattered at an angle smaller than the total internal
reflection angle is emitted from the optical element to the outside
(air).
[0028] Herein, in a case where no minute regions are dispersedly
distributed in the translucent resin, since such selective
scattering of polarized light does not occur, of the light
resulting from excitation by the luminous body in the optical
element, about 80% of light is confined within the translucent
resin and repeats the total reflection on the relationship with the
solid angle.
[0029] According to the invention, the light confined within the
optical element is emitted to the outside of the optical element
only in a case where the total reflection condition has been broken
due to scattering at the interface between the minute regions and
the translucent resin. Thus, it is possible to optionally control
the light emission efficiency according to the size of each minute
region, distribution ratio of the minute regions, or the like.
[0030] On the other hand, light scattering at an angle larger than
the total reflection angle in the above .DELTA.n1 direction, light
colliding with no minute regions, and light having a plane of
vibration in a direction other than the .DELTA.n1 direction are
confined within the optical element and transmitted therethrough as
repeating the total reflection, with waiting a chance for emission
by eliminating a polarized state owing to the birefringent phase
difference or the like within the optical element and allowing
light itself to meet the .DELTA.n1 direction condition (that is,
turn into linearly polarized light having a plane of vibration
parallel to the .DELTA.n1 direction). These steps are thus repeated
and, as a result, linearly polarized light having a predetermined
plane of vibration is emitted from the optical element in an
efficient manner.
[0031] Here, when the particle size of the luminous body is larger
than a predetermined one, as shown in FIG. 1A, linearly polarized
light (linearly polarized light having a plane of vibration
parallel to the .DELTA.n1 direction) L, which results from
excitation by one luminous body in the optical element and is
obtained by colliding with the minute regions, meets the conditions
for emission to the outside but is scattered and depolarized
through collision with other luminous body before emission to the
outside of the optical element. As a result, there is a possibility
that the degree of polarization of the emitted light is lowered.
Particularly, in this case, since length of optical path of light
which is transmitted within the optical element in the form of the
above linearly polarized light is relatively long and
reflection/scattering are repeated two or more times, probability
of colliding with other luminous body 3 before emission to the
outside of the optical element is high. However, according to the
invention, since the particle size of the luminous body is smaller
than the emission wavelength (visible light region) thereof
(therefore, smaller than the wavelength of the linearly polarized
light L), as shown in FIG. 1B, the linearly polarized light L is
hardly scattered by other luminous body 3 and passed through, so
that a possibility of depolarization hardly exists. Namely, since
light has properties as a wave, it passes through without being
affected by objects smaller than its wavelength in most cases.
Accordingly, the linearly polarized light can be emitted as
linearly polarized light having a sufficient degree of
polarization.
[0032] Moreover, since the particle size of the luminous body is
smaller than its emission wavelength, the particle size of the
luminous body is sufficiently small as compared with a practically
assumed thickness of the optical element and hence defective
appearance of protrusion of dispersed luminous body from the
optical element surface does not occur. Also, at the preparation of
the optical element, the luminous body may not be an obstruction
for formation of the minute regions nor a starting point of
breakage of the translucent resin when stretching is performed, so
that its preparation is facilitated.
[0033] Furthermore, since the particle size of the luminous body is
smaller than its emission wavelength, the luminance of light
emitted from the optical element can be effectively enhanced. As
shown in FIG. 2, this is because reduction of the particle size of
the luminous body 3 to be dispersed (FIG. 2A) allows the luminous
body 3 to be dispersed in a larger number as compared with the case
of a large particle size (FIG. 2B) even when the same total weight
of the luminous body is dispersed in the optical element. For
example, under the condition of the same total weight, when the
particle size of the luminous body 3 is reduced to one second, the
total number of the luminous body 3 becomes eight times and the
total surface area of the luminous body 3 becomes twice. Since
excitation of the luminous body 3 occurs on the surface of the
luminous body 3, enlargement of the total surface area for whole
number of the luminous body 3 increases quantity of emitted light
by just that much and, as a result, it is possible to effectively
enhance the luminance of light emitted from the optical
element.
[0034] As mentioned above, according to the invention, light
resulting from excitation by incident light can be emitted to the
outside in the form of linearly polarized light having a sufficient
degree of polarization through at least one of front and rear
sides, the optical element can be easily prepared without
occurrence of defective appearance, and the luminance of emitted
light can be easily enhanced.
[0035] Preferably, the above luminous body is an inorganic
pigment.
[0036] According to such an arrangement, an inorganic pigment
exhibits a high luminance of emitted light (emission efficiency)
and also has an extremely high durability, so that it can be
durable to long-term use. Therefore, it is possible to obtain an
optical element excellent in luminance of emitted light,
durability, and reliability as compared with the case using a
dye-based luminous body.
[0037] The above luminous body is preferably a fluorescent pigment
that absorbs ultraviolet light or visible light and emits visible
light.
[0038] Alternatively, the above luminous body may be a
phosphorescent pigment that absorbs ultraviolet light or visible
light and emits visible phosphorescence.
[0039] In order to further reduce a possibility of depolarization,
the particle size of the above luminous body is preferably not more
than one fifth of the emission wavelength of the luminous body. The
particle size of the luminous body is more preferably not more than
one tenth of the emission wavelength of the luminous body, and
further preferably not more than one fiftieth of the emission
wavelength of the luminous body.
[0040] Here, in a case where the dispersed luminous body is
aggregated to form an aggregate, the aggregate shows a behavior
similar to the luminous body having a particle size equal to the
diameter of the aggregate (see FIG. 1C). Therefore, the diameter of
the aggregate formed by aggregating the above luminous body is
preferably smaller than the emission wavelength of the luminous
body. The diameter of the aggregate formed by aggregating the above
luminous body is more preferably not more than one fifth of the
emission wavelength of the luminous body, and further preferably
not more than one tenth of the emission wavelength of the luminous
body.
[0041] Preferably, the minute regions are made of a liquid
crystalline material; a glass state material formed by cooling and
fixing a liquid crystal phase; or a material Conned by crosslinking
and fixing a liquid crystal phase of a polymerizable liquid crystal
with an energy ray.
[0042] Alternatively, the minute regions may be made of a liquid
crystal polymer that has a glass transition temperature of
50.degree. C. or higher and exhibits a nematic liquid crystal phase
at a temperature lower than the glass transition temperature of the
above translucent resin.
[0043] Preferably, the following relations are satisfied:
0.03.ltoreq..DELTA.n1.ltoreq.0.5 0.ltoreq. 6n2=0.03
0.ltoreq..DELTA.n3.ltoreq.0.03 where, .DELTA.n1 is refractive index
difference between the minute regions and the translucent resin in
an axial direction of the minute regions, along which a value of
the restive index difference between the minute regions and the
translucent resin occurs; and .DELTA.n2 and .DELTA.n3 are the
refractive index differences in an anal direction orthogonal to the
axial direction along which the maximum refractive index difference
occurs, respectively.
[0044] Incidentally, when a material absorbing relatively much
light having the wavelength of excitation light is used as the
translucent resin or the minute regions, the material absorbs the
excitation light and hence emission efficiency tends to be lowered.
Furthermore, when ultraviolet light is used as the excitation
light, deterioration of the material may be invited owing to the
absorption of ultraviolet light. Thus, the use of a material
substantially absorbing no light having the wavelength of
excitation light as a material of the translucent resin or the
minute regions can reduce decrease in emission efficiency and
deterioration of the material as far as possible. For example, in a
case where the excitation light is ultraviolet light, both of the
translucent resin and the minute regions are preferably made of
materials that do not substantially absorb ultraviolet light. In
this connection, the range of the wavelength band of the
ultraviolet light may be a range commonly recognized as the
wavelength band of ultraviolet light and may be the range of about
1 to 400 nm, for example. Moreover, the term "substantially absorb
no ultraviolet light" means no absorption of ultraviolet light and
also means that light absorption rate at the wavelength of
excitation light is about 40% or less even when ultraviolet light
is absorbed.
[0045] Also, according to the invention, there is provided a
polarized-light emitting planar light source that includes the
above optical element of the invention and an excitation light
source that emits light of a wavelength that is capable of exciting
a luminous body dispersed in the optical element.
[0046] Additionally, according to the invention, there is also
provided a polarized-light-emitting planar light source, wherein
the translucent resin and the minute regions are made of materials
that substantially absorb no ultraviolet light and the light of a
wavelength that is capable of exciting the luminous body dispersed
in the optical element is ultraviolet light.
[0047] Preferably, the polarized-light-emitting planar light source
firer includes a light guide member for guiding light emitted from
the excitation light source to the optical element, the light guide
member being made of a translucent material.
[0048] The exciting light source may be composed of an inorganic or
organic electroluminescent element or a mercury-free fluorescent
tube.
[0049] Furthermore, according to the invention, there is provided a
display device that includes the above polarized-light-emitting
planar light source.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0050] According to the present invention, light resulting from
excitation by incident light can be emitted to the outside in the
form of linearly polarized light having a sufficient degree of
polarization through at least one of front and rear sides, an
optical element can be easily prepared without occurrence of
defective appearance, and the luminance of emitted light can be
easily enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 is a schematic view for illustrating influence of the
particle size of a luminous body on scattering of light.
[0052] FIG. 2 is a schematic view for illustrating influence of the
particle size of a luminous body on luminance of emitted light.
[0053] FIG. 3 is a vertical cross sectional view illustrating a
schematic structure of an optical element according to one
embodiment of the invention.
[0054] FIG. 4 is a vertical cross sectional view illustrating a
schematic structure of a polarized-light-emitting planar light
source, to which an optical element according to one embodiment of
the invention has been applied.
[0055] FIG. 5 is a vertical cross sectional view partially
illustrating a schematic structure of the polarized-light-emitting
planar light source shown in FIG. 4 in a case where a different
excitation light source is used.
[0056] FIG. 6 is a schematic view for explaining the fact that
uniform light emission is apt to be obtained even if the excitation
light source is a point source when an optical element according to
one embodiment of the invention has been applied.
DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS
[0057] 1. translucent resin
[0058] 2. minute regions
[0059] 3. light-emitting material
[0060] 4. translucent sheet
[0061] 5. reflection layer
[0062] 6. light diffusion layer
[0063] 7. lens sheet
[0064] 8. adhesive layer
[0065] 9. excitation light source
[0066] 10. optical element
BEST MODE FOR CARRYING OUT THE INVENTION
[0067] An embodiment according to the present invention will be
hereinafter described with reference to the accompanying
drawings.
[0068] FIG. 3 is a vertical cross sectional view illustrating a
schematic structure of an optical element according to one
embodiment of the invention. As illustrated in FIG. 3, an optical
element 10 according to this embodiment has a translucent resin 1
and minute regions 2 that are dispersedly distributed in the
translucent resin 1 and have a birefringence different from the
translucent resin 1, and is formed into a plate-like shape. The
optical element 10 contains at least one luminous body 3 in the
translucent resin 1 and/or the minute regions 2. FIG. 3A shows an
example where the luminous body 3 is dispersed in the translucent
resin 1, FIG. 3B shows an example where the luminous body 3 is
dispersed in the minute regions 2, and FIG. 3C shows an example
where the luminous body 3 is dispersed in both of the translucent
resin 1 and the minute regions 2. The optical element 10 according
to this embodiment may be any of the arrangements of FIG. 3A to
FIG. 3C.
[0069] The optical element 10 is not necessarily formed into a
specific shape, as far as it has two flat sides oppositely located
to each other. However, in view of the possibility of application
to a planar light source or total reflection efficiency, it is
preferable to form the optical element into a film-like, sheet-like
or plate-like shape having a rectangular cross section as shown in
FIG. 3. Particularly, the optical element 10 having a plate like
shape is advantageous for ease of handling. The term "plate-like"
in the invention is a concept including all these film-like,
sheet-like and plate-like shapes.
[0070] The optical element 10 has a thickness of preferably 20
.mu.m to 3 mm, more preferably 30 .mu.m to 1 mm, further preferably
40 .mu.m to 500 .mu.m, and particularly preferably 50 .mu.m to 200
.mu.m. When the thickness of the optical element is less than 20
.mu.m, there is a possibility of occurrence of uneven luminance
because excitation light emitted from the excitation light source
may directly pass through or scattering ability at the minute
regions 2 may be impaired. Also, since transmission path of the
scattered light at the minute regions 2 is not sufficiently
secured, there is a possibility that linearly polarized-light
having a sufficient degree of polarization is not obtained. On the
other hand, when the thickness of the optical element 10 is more
than 3 mm, excitation light is not sufficiently transmitted in a
thickness direction of the optical element 10 and all the luminous
bodies dispersed cannot be effectively used, so that there is a
possibility of decreasing emission efficiency. Therefore, the above
thickness is preferred.
[0071] Opposite sides 101, 102 (FIG. 3A) of the optical element 10
each preferably has a surface smoothness similar to a mirror
surface in view of a light confining efficiency that contributes to
the ability to confine light which is formed by the luminous body
3, within the optical element 10 by total reflection. When the
opposite sides 101, 102 of the optical element 10 have poor surface
smoothness, a translucent film or sheet having excellent surface
smoothness may be bonded to the translucent resin 1 with a
transparent adhesive or a pressure-sensitive adhesive so as to make
the smooth surface of the bonded film or sheet act as a total
reflection interface, thereby the same effect as above is also
obtained.
[0072] Preferably, the luminous body 3 is homogeneously dispersed
into either or both of the translucent resin 1 and the minute
regions 2. As mentioned above, when light scattering by the
luminous body 3 occurs, there is a possibility of depolarization,
so that the particle size of the luminous body 3 according to this
embodiment is smaller than the emission wavelength thereof. In
order to further reduce a possibility of depolarization, the
particle size of the above luminous body 3 is preferably not more
than one fifth, more preferably not more than one tenth, and
further preferably not more than one fiftieth of the emission
wavelength of the luminous body.
[0073] By controlling the particle size of the luminous body 3 to a
size at which a quantum effect occurs (specifically about 1 to 10
nm), luminous bodies 3 having different emission wavelengths
depending on the particle sizes can be prepared even when the
luminous bodes have the same composition. Therefore, when the
luminous bodies 3 having different emission wavelengths depending
on the particle sizes are used (luminous bodies having different
particle sizes are suitably combined), a broad emission wavelength
band can be obtained by suitably controlling the particle size
distribution of luminous bodies 3 having the same composition
without using plurality of luminous bodies 3 having different
compositions. The particle size of the luminous body 3 can be
measured using a dynamic light scattering particle size
distribution-measuring apparatus manufactured by Otsuka Electronics
Co., Ltd. or Horiba Ltd. or a laser zeta-potential electrometer and
also can be measured by direct observation on an electron
microscope or by flying time measurement proposed by Tsukuba
Nano-technology. Also, in a case where a large mass of raw material
of the luminous body 3 is pulverized to obtain the luminous body 3,
it is possible to obtain a luminous body 3 having a desired
particle size by controlling pulverizing conditions (time, rotation
sate, pressure, temperature, etc.) or classification through
filtration or precipitation after pulverization. Moreover, in a
case where the luminous body 3 is obtained by growing atoms and
molecules, a luminous body 3 having a desired particle size can be
obtained by controlling growth conditions (concentration of
dispersion liquid, temperature, feeding rate of raw materials,
etc.). Furthermore, in a case where a luminous body 3 is obtained
by spattering with electron beam in a rare gas using a raw material
of the luminous body 3 as a target, it is possible to obtain a
luminous body 3 having a desired particle size by controlling power
of the electron beam, kind and concentration of the rare gas,
nature of the target, and the like.
[0074] Moreover, in a case where an aggregate is formed through
aggregation of dispersed luminous body 3, the aggregate shows a
behavior similar to the luminous body having a particle size equal
to the diameter of the aggregate. Therefore, the diameter of
aggregate formed by aggregating the luminous body 3 is preferably
smaller than the emission wavelength of the luminous body 3. The
diameter of the aggregate formed by aggregating the above luminous
body 3 is more preferably not more than one fifth, and ether
preferably not more than one tenth of the emission wavelength of
the luminous body 3. The diameter of the above aggregate can be
measured by the methods similar to the above methods for measuring
the particle size of the luminous body 3 itself. Moreover, it is
possible to suppress the aggregation of the luminous body 3 by
attaching a coupling agent or a surfactant to the surface of the
luminous body 3 to electrostatically charge the surface of the
luminous body 3.
[0075] As the luminous body 3, one or more of suitable materials,
which absorb ultraviolet light or visible light and emit light
having a wavelength in visible light region upon excitation, can be
used. In the invention, the luminous body is preferably an
inorganic pigment. An inorganic pigment exhibits a high luminance
of emitted light and also has an extremely high durability, so that
it can be durable to long-term use. Therefore, it is possible to
obtain an optical element 10 excellent in luminance of emitted
light, durability, and reliability as compared with the case using
a dye-based luminous body. More specifically, it is preferred to
use a fluorescent pigment composed of an inorganic pigment
radiating fluorescence that is light emitted from singlet excited
state, a phosphorescent pigment radiating phosphorescence that is
light emitted from triplet excited state, or the like.
[0076] When more specifically described, as the luminous body 3,
suitably used are CdSe, ZnS, Y.sub.2O.sub.5S, LaPO.sub.4,
Ca.sub.10(PO.sub.4).sub.6FCl, (SrCaBaMg).sub.5(PO.sub.4).sub.3Cl,
BaMgAl.sub.10O.sub.17, Zn.sub.2SiO.sub.4, (Y,Gd)BO.sub.3, ZnSe,
CdSe, ZnTe, CdTe, etc. and also those obtained by doping them with
a metal such as Ce, Tb, Eu, Al, Sb, or Mn or a rare-earth
element.
[0077] The refractive index of an inorganic-pigment is generally
2.0 or more and the pigment is opaque and colored in many cases.
For example, CdSe shows coloring of red to orange although it
depends on particle size and purity. In a case where such an
inorganic pigment is used as the luminous body 3, depolarization of
light emitted from excitation by scattering caused by large
refractive index difference between the luminous body 3 and a resin
for dispersing the same (translucent resin 1 or material forming
minute regions 2 and most of them has a refractive index of 1.5 to
1.7) and coloration of light emitted from excitation caused by
absorption induced by opacity and coloring of the inorganic pigment
itself generally become problems. However, as mentioned above,
since the luminous body 3 according to this embodiment has a
particle size smaller than the emission wavelength thereof, most of
light emitted from excitation directly passes through without being
affected by the luminous body 3, so that the above problems hardly
occur.
[0078] The luminous body 3 can be dispersed in the optical element
10 by a suitable method, such as a method of blending the luminous
body 3 prepared beforehand with the translucent resin 1 and a
material forming the minute regions 2 together with other
additive(s) according to need at the preparation of the optical
element 10 or a method of blending raw material of the luminous
body 3 beforehand and subsequently precipitating the luminous body
3 by carrying out thermal treatment, optical treatment, oxidative
treatment, reductive treatment, acid-base reaction treatment, or
the like.
[0079] When more specifically described, it is possible to use the
methods shown in the following (1) to (4). Namely,
[0080] (1) a method of dissolving raw materials of the luminous
body 3 with an acid/base or the like, impregnating the solution in
a resin (translucent resin 1 or material forming minute regions 2)
such as polyvinyl alcohol, and removing the component of the
acid/base dissolving the above raw material by treatment after film
formation;
[0081] (2) a method of dispersing a solution containing a metal ion
(e.g., Zn ion) protected by chelation dissolved therein into a
resin (translucent resin 1 or material forming minute regions 2),
subsequently canceling the chelation, and adding a necessary ion (a
sulfide ion obtained from an aqueous Na.sub.2S solution or H.sub.2S
gas) to the precipitating metal ion to growth the luminous body
3;
[0082] (3) a method of reacting an organometallic compound (e.g., a
reaction product of an organic acid such as acetic acid, benzoic
acid, formic acid, butyric acid, tartaric acid, lactic acid, or
oxalic acid with a metal ion) with an organophosphorus compound
(e.g., a phosphate ester), forming a cluster by thermally
decomposing the organometallic compound to form the luminous body
3, and dispersing the formed luminous body 3 into a resin
(translucent resin 1 or material forming minute regions 2);
[0083] (4) a method of adding a surfactant solution to an aqueous
solution containing an metal ion dissolved therein to form a
cluster and growing the luminous body 3 by refluxing the whole
under reductive conditions; or the like method may be suitably
employed.
[0084] The optical element 10 can be made by various methods such
as by producing an oriented film under an appropriate molecular
orientation through a stretching treatment of one or more materials
having an excellent transparency such as a polymer and/or a liquid
crystal in such a combination as to form regions having
birefringences different from each other (minute regions). As
mentioned above, since the luminous body 3 is dispersed in the
optical element 10, it is preferable that at least one of the
combined materials can be incorporated into the luminous body 3 to
be dispersed, with good compatibility.
[0085] As examples of the combination of materials, it can be cited
a combination of a polymer and a liquid crystal, a combination of
an isotropic polymer and an anisotropic polymer, a combination of
anisotropic polymers, etc. In order to achieve even distribution of
the minute regions 2, the combination enabling phase separation is
preferable. Also, the distribution of the minutes regions 2 can be
controlled on the basis of the compatibility of the combined
materials. For example, the phase separation can be achieved by
various methods such as a method of bringing incompatible materials
into solution by a solvent, or a method of heat-melting
incompatible materials and mixing them together under molten
state.
[0086] The mixing ratio of the luminous body 3 is not particularly
limited but a necessary quantity of emitted light cannot be
obtained when the mixing ratio is too small. Therefore, the mixing
ratio of the luminous body 3 is preferably 0.1% by weight or more,
more preferably 0.5% or more, and further preferably 1.0% by weight
or more. Contrarily, when the mixing ratio of the luminous body 3
is too large, stretching and phase separation of an orientation
base material (translucent resin 1 or material forming minute
regions 2) may be influenced, so that the mixing ratio may be
suitably determined within the range resulting in no such
influence. An upper limit of the mixing ratio is preferably 10% by
weight or less, and more preferably 5% by weight or less.
[0087] In a case where the molecular orientation is made by
subjecting the above combination of materials to the stretching
treatment, the optical element 10 suitable for each application or
purpose can be formed by appropriately setting a stretching
temperature and stretching ratio for the combination of a polymer
and a liquid crystal and a combination of an isotropic polymer and
an anisotropic polymer, or by appropriately controlling the
stretching conditions for the combination of anisotropic polymers.
While anisotropic polymers are classified into positive and
negative based on a characteristics of refractive index variation
by the stretching direction, any one of positive and negative
anisotropic polymers can be used in this embodiment. Accordingly,
the combination of positive anisotropic polymers, the combination
of negative polymers, and the combination of positive and negative
polymers are all possible to use.
[0088] As examples of the above polymers, there may be mentioned
ester polymers such as polyethylene terephthalate and polyethylene
naphthalate, styrene polymers such as polystyrene and
acrylonitrile-styrene copolymer (AS polymers), olefin polymers such
as polyethylene, polypropylene, cyclic polyolefine and polyolefins
having a norbornene structure, and ethylene/propylene copolymer,
acrylic polymers such as polymethyl methacrylate, cellulose
polymers such as cellulose diacetate and cellulose triacetate, and
amide polymers such as nylon and aromatic polyamides.
[0089] As examples of the above transparent polymer, there may be
also mentioned carbonate polymers, polyvinyl chloride polymers,
imide polymers, sulfone polymers, polyether sulfone, polyether
ether ketone, polyphenylene sulfide, vinyl alcohol polymers,
vinylidene chloride polymers, vinyl butyral polymers, acrylate
polymers, polyoxymethylene, silicone polymers, urethane polymers,
ether polymers, vinyl acetate polymers or their mixtures, and
thermosetting- or UV-curing polymers such as phenolic, melamine,
acrylic, urethane, acrylic urethane, epoxy or silicone
polymers.
[0090] On the other hand, as examples of the above liquid crystal,
there may be mentioned low-molecular-weight liquid crystals and
crosslinkable liquid crystal monomers such as cyanobiphenyl,
cyanophenylcyclohexane, cyanophenyl ester, phenyl benzoate ester or
phenylpyrimidine liquid crystals or their mixtures, which exhibit a
nematic phase or smectic phase at room temperature or high
temperature, as well as liquid crystal polymers, which exhibit a
nematic phase or smectic phase at room temperature or high
temperature. The above crosslinkable liquid crystal monomers are
usually subjected to a molecular orientation treatment, and then
crosslinked into polymers by an appropriate method including the
application of heat, light, or the like.
[0091] In order to produce the optical element 10 having an
excellent heat resistance and durability, it is preferable to use
the combination of a polymer having a glass transition temperature
of preferably 50.degree. C. or higher, more preferably 80.degree.
C. or higher and particularly preferably 120.degree. C. or higher
and a crosslinkable liquid crystal monomer or a liquid crystal
polymer. An upper limit of the glass transition temperature of the
above polymers is preferably 300.degree. C. or lower, more
preferably 250.degree. C. or lower, and further preferably
200.degree. C. or lower. As the above liquid crystal polymer, a
main-chain type or side-chain type polymer or the like is
appropriately used without particular limitation in type. It is
preferable to use a liquid crystal polymer having a polymerization
degree of preferably 8 or higher, more preferably 10 or higher, and
particularly preferably 15 to 5000 in view of contribution to the
formation of the minute regions 2 with an excellent homogeneous
particle size distribution, as well as thermal stability, film
formability easiness of molecular orientation, and the like.
[0092] The optical element 10 using a liquid crystal polymer can be
formed by various methods such as a method of mixing one or more of
polymers with one or more of liquid crystal polymers for forming
the minute regions 2, thereby forming a polymer film containing the
liquid polymer dispersedly distributed to occupy the minute
regions, and subjecting the polymer film to molecular orientation
by a suitable method, and thereby forming regions having different
birefringences.
[0093] Herein, with respect to the refractive index difference
between the minute regions 2 and the translucent resin 1, the
refractive index difference in an axial direction of the minute
regions 2, along which a maximum refractive index difference
occurs, is represented by .DELTA.n1, and the refractive index
differences in directions respectively orthogonal to the axial
direction along which the maximum refractive index difference
occurs are respectively represented by .DELTA.n2 and .DELTA.n3. In
view of controllability of the refractive index differences by the
above molecular orientation, the above liquid crystal polymer has
preferably a glass transition temperature of 50.degree. C. or
higher and exhibits a nematic phase in a temperature range lower
than the glass transition temperature of the polymer (translucent
resin 1) simultaneously used. An upper limit of the glass
transition temperature of the above liquid crystal polymer is
preferably 250.degree. C. or lower, more preferably 200.degree. C.
or lower, and further preferably 150.degree. C. or lower. As a
specific example thereof, there may be mentioned a side-chain type
liquid crystal polymer having a monomer unit represented by the
following general formula: General formula: ##STR1##
[0094] In the above general formula, X represents a backbone group
which constitutes the main chain of the liquid crystal polymer, and
may be formed by appropriate linking chains such as linear,
branched or cyclic groups. As specific examples thereof, there may
be mentioned polyacrylates, polymethacrylates,
poly(.alpha.-haloacrylate)s, poly(.alpha.-cyanoacrylate)s,
polyacrylamides, polyacrylonitriles, polyphthacrylonitriles,
polyamides, polyesters, polyurethanes polyethers, polyimides, and
polysiloxanes.
[0095] Moreover, Y represents a spacer group branching from the
main chain. As examples of the spacer group Y to achieve the
formidability of the optical element 10 including control of
refractive index difference, there may be preferably mentioned
ethylene, propylene, butylenes, pentylene, hexylene, octylene,
decylene, undecylene, dodecylene, octadecylene, ethoxyethylene, and
methoxybutylene. On the other hand, Z represents a mesogen group
which imparts liquid crystal alignment properties.
[0096] The above side-chain type liquid crystal polymers to be
aligned in nematic orientation may be any appropriate thermoplastic
polymers such as homopolymers or copolymers having monomer units
represented by the above general formula. Of these, those having an
excellent property in monodomain orientation are preferable.
[0097] The optical element 10 using a liquid crystal polymer to be
aligned in nematic orientation may be formed by, for example, a
method that includes: mixing a polymer for forming a polymer film
with a liquid crystal polymer that exhibits a nematic phase in a
temperature range lower than the glass transition temperature of
the polymer and has a glass transition temperature of preferably
50.degree. C. or higher, more preferably 60.degree. C. or higher
and particularly preferably 70.degree. C. or higher, thereby
forming a polymer film containing the liquid crystal polymer
dispersedly distributed so as to occupy the minute regions 2,
heating the liquid crystal polymer, which is to form the minute
regions 2, to align the same in nematic orientation; and fixing the
orientation state by cooling. An upper limit of the glass
transition temperature of the above liquid crystal polymer is
preferably 250.degree. C. or lower, more preferably 200.degree. C.
or lower, and further preferably 150.degree. C. or lower.
[0098] A polymer film (translucent resin 1) containing the minute
regions 2 dispersedly distributed therein before orientation, that
is, a film to be oriented may be formed by an appropriate method
such as a casting method, extrusion molding method, injection
molding method, roll forming method, flow casting method or the
like. It is also possible to form a film by spreading a monomer
mixture and polymerizing the spread mixture by heating or
irradiation with ultraviolet light or the like.
[0099] In view of producing the optical element 10 containing the
minute regions 2 excellent in even distribution therein, a film
forming method, in which a mixed solution of materials is formed
into a film using a solvent by a casting method or a flow casing
method, is preferably employed. In such a case, the size and
distribution of the minute regions 2 can be controlled by changing
the type of the solvent, viscosity of the mixed solution, or drying
speed of a layer formed by spreading the mixed solution. The
decrease in viscosity of the mixed solution, increase in drying
speed of the mixed solution spread layer or the like is effective
in reducing the area of the minute regions 2.
[0100] While the thickness of the film to be oriented may be
appropriately determined, in general, it is preferably set in the
range of 10 mm or less, more preferably 30 .mu.m to 5 mm, further
preferably 50 .mu.m to 2 mm, and particularly preferably 100 .mu.m
to 1 mm in view of easiness of orientation. In forming the film, it
is possible to incorporate appropriate additives such as a
dispersant, a surfactant, a color tone regulator, a flame
retardant, a release agent, and an antioxidant.
[0101] The orientation of the film can be made, for example, by
employing one or more methods capable of controlling the refractive
index by the orientation, such as a uniaxial, biaxial, successive
biaxial or Z-axis stretching method; a rolling method; a method of
applying an electric field or magnetic field at a temperature
higher than the glass transition temperature or liquid crystal
transition temperature and sharply cooling to fix the orientation;
a method of flow orientation during film forming process; or a
method of self-orientation of a liquid crystal on the basis of a
slight orientation of an isotropic polymer. Therefore, the optical
element 10 produced may be in the form of a stretch film or
non-stretched film. For a stretch film, while a fragile polymer may
be used, a polymer having an excellent stretchability is preferably
used. Moreover, in a case where the thickness of the film to be
oriented is 2 mm or more, a suitable orientation can be achieved
using a rolling method as the stretching method.
[0102] In a case where the minute regions 2 are made of a liquid
crystal polymer, the orientation can be achieved, for example, by
heating a polymer film to such a temperature as to enable a liquid
polymer dispersedly distributed therein to exhibit a target liquid
crystal phase such as a nematic liquid phase and turn into a molten
state, applying orientation by the action of an orientation
regulation force, and then sharply cooling the film, thereby fixing
the orientation. The orientation of the minute regions 2 is
preferably held in a monodomain state in view of preventing
fluctuation in optical characteristics or the like.
[0103] As the orientation regulation force, a stretching force
available in a process of allowing a polymer film to be stretched
by an appropriate ratio, a shearing force in a film forming
process, an electric field or a magnetic filed, which are all
capable of orienting the liquid crystal polymer, is applicable. One
or more of these orientation regulation forces may be applied to
achieve an appropriate orientation of the liquid crystal
polymer.
[0104] A region of the optical element 10 other than the minute
regions 2, that is, the translucent resin 1 may possess
birefringent or isotropic characteristics. The optical element 10,
which exhibits birefringent characteristics in its entire region,
can be produced by the molecule orientation in the aforementioned
film forming process using a birefringent polymer as a film forming
material. According to needs and desires, a known orientation
method such as a stretching method is applied so that the
birefringent characteristics can be imparted or controlled. The
optical element 10, in which a region other than the minute regions
2 has isotropic characteristics, can be produced by a method of
stretching a film derived firm an isotropic polymer used as a film
forming material in a temperature range lower than the glass
transition temperature of the polymer.
[0105] As mentioned above, the translucent resin 1 is different in
birefringent characteristics from the minute regions 2.
Specifically, as mentioned above, with respect to the refractive
index difference between the minute regions 2 and the translucent
resin 1, when the refractive index difference of the minute regions
2 in an axial direction (a .DELTA.n1 direction), along which a
maximum refractive index difference occurs, is designated as
.DELTA.n1, and the refractive index differences in axial directions
(.DELTA.n2 and .DELTA.n3 directions) orthogonal to the axial
direction, along which the maximum refractive index difference
occurs, are respectively designated as .DELTA.n2 and .DELTA.n3, it
is preferable to have a suitably large .DELTA.n1, while preferably
keeping .DELTA.n2 and .DELTA.n3 as small as possible or as close as
possible to 0, in view of the total reflection to be mentioned
below. The optical element 10 according to this embodiment is
controlled so as to preferably have
0.03.ltoreq..DELTA.n1.ltoreq.0.5, 0.ltoreq..DELTA.n2.ltoreq.0,03,
0.ltoreq..DELTA.n3.ltoreq.0.03, and more preferably
.DELTA.n2=.DELTA.n3. These refractive index differences can be
controlled by the refractive index of a material used, an
orientation method, or the like.
[0106] With the refractive index differences .DELTA.n1, .DELTA.n2
and .DELTA.n3 as set above, of the light resulting from excitation
by excitation light entering the optical element 10, linearly
polarized light in the .DELTA.n1 direction is strongly scattered at
an angle smaller than an critical angle (a total reflection angle),
so that the quantity of light emitted from the optical element 10
to the outside can be increased, while linearly polarized light in
directions other than the .DELTA.n1 direction is hard to be
scattered, thus repeating the total reflection. As a result, the
linearly polarized light in directions other than the .DELTA.n1
direction can be confined to the inside of the optical element
10.
[0107] The refractive index difference between each of the axial
directions (.DELTA.n1, .DELTA.n2 and .DELTA.n3) of the minute
regions 2 and the translucent resin 1 represents the average
refractive index difference between the respective axial directions
of the minute regions 2 and the translucent resin 1 in the case of
the translucent resin 1 having optically isotropic characteristics.
Moreover, in the case of the translucent resin 1 having optically
anisotropic characteristics, the above refractive index difference
represents the refractive index difference in each axial direction,
since the direction of the principal light axis of the translucent
resin 1 is usually identical with the direction of the principal
light axis of the minute regions 2.
[0108] Since the .DELTA.n1 direction is parallel to a plane of
vibration of linearly polarized light emitted from the optical
element 10, the .DELTA.n1 direction is preferably parallel to the
opposite two sides 101, 102 of the optical element 10. As far as
the .DELTA.n1 direction is parallel to the two sides 101, 102, the
direction may be any direction suitable for a liquid crystal cell
or the like to which the optical element 10 is applied.
[0109] In view of obtaining a higher homogeneity of the scattering
effect or the like in the minute regions 2, it is preferable to
have the minute regions 2 dispersedly distributed as evenly as
possible in the optical element 10. The size of each minute region
2, particularly the length in the scattering direction, i.e., the
.DELTA.n1 direction affects backscattering (reflection) or
wavelength dependency. In order to improve the light utilization
efficiency, prevent coloring due to the wavelength dependency,
prevent deterioration in visual recognition due to visualization of
the minute regions 2 or deterioration in clear display, or obtain
an improved film formability or film strength, the size of each
minute region 2, particularly the length in the .DELTA.n1 direction
is preferably in the range of 0.05 to 500 .mu.m, more preferably
0.1 to 250 .mu.m, and particularly preferably 1 to 100 .mu.m. The
minute regions 2 usually exist in the optical element 10 in a
domain state and its length in the .DELTA.n2 direction or the like
is not particularly limited.
[0110] While the ratio of the minute regions 2 occupying the inside
of the optical element 10 may be appropriately determined in
consideration of the scattering characteristics in the .DELTA.n1
direction or the like, it is generally set to preferably 0.1 to 70%
by weight, more prefrably 0.5 to 50% by weight, and particularly
preferably 1 to 30% by weight in view of film strength or the
like.
[0111] The optical element 10 according to this embodiment can form
a polarized-light-emitting planar light source in combination with
a light source that emits light having a wavelength capable of
exciting the luminous body 3 dipersed in the optical element 10.
While the arrangement of the light source and the optical element
10 is not particularly limited, it is desirable to employ an
arrangement allowing excitation light to effectively enter the
optical element 10. From such a viewpoint, as illustrated in FIG.
4, it is preferable to employ an arrangement with an excitation
light source 9 located on a lateral side of the optical element 10,
or an arrangement where the excitation light source 9 is a planar
light source such as an electroluminescent element and one of the
flat sides of the optical element 10 is positioned opposite to an
upper side of the planar light source, as illustrated in FIG. 5.
The optical element 10 may be independently arranged as illustrated
in FIG. 4, or arranged integrally with the excitation light source
9 and/or a translucent support member via a translucent adhesive
layer. For producing a more efficient result, a light guiding
member for guiding light from the excitation light source into the
optical element 10 is preferably provided. The light guiding member
is not particularly limited and there may be suitably used those
commonly used for back light of liquid crystal displays, such as
light guiding plates having a flat plate shape or wedge shape made
of a translucent resin and light guiding plates made of the
translucent resin containing reflective dots.
[0112] The type of the excitation light source 9 is not
particularly limited as far as it is an excitation light source,
which emits light having a wavelength capable of exciting the
luminous body 3. Since the luminous body 3 emits light basically
through conversion of a short-wavelength light having a high energy
into a long-wavelength light, it is preferable to use an excitation
light source emitting ultraviolet light or an excitation light
source having an emission band of visible light to ultraviolet
light. For example, in a case where an excitation light source
emitting visible light is used as the excitation light source 9,
when visible light itself, which is excitation light, is
transmitted, color reproduction tends to be inhibited.
Particularly, in a case of preparing white light, the transmittance
of light from the excitation light source should be also considered
and hence the setting becomes complex. However, when an excitation
light source emitting ultraviolet light is used as the excitation
light source 9, even in a case where the ultraviolet light is
transmitted, the light is not visible and hence it is not necessary
to consider the transmittance of light from the excitation light
source in the setting. Moreover, as in the case of white light
formation for light-emitting diode (LED), using blue visible light
as excitation light and a yellow fluorescent body
YAG:Ce=cerium-incorporated yttrinum aluminum garnet) as the
luminous body 3, apparent white light may be formed using the
emitted light from the yellow fluorescent body and transmitting
excitation light but the apparent white light is poor in color
reproduction since it lacks red color component. Therefore, in
order to obtain true white light, it is preferred to use a luminous
body 3, which emits light consisting of three primary colors such
as R (red color)/G (green color)/B (blue color), and it is desired
to use an excitation light source emitting ultraviolet light of a
short-wavelength side having a high energy as mentioned above as
the excitation light source 9 emitting light having a wavelength
capable of exciting the optical element 3, which emits light
consisting of such three primary colors.
[0113] When more specifically described, as the excitation light
source 9 according to this embodiment, there may be suitably used
conventional ultraviolet to visible light-emitting light sources
using mercury vapor, such as hot cathode fluorescent tubes and cold
cathode fluorescent tubes, and also mercury-free fluorescent tubes
using environmentally-friendly substances such as xenon gas,
manufactured and sold by Sanyo Electric Co., Ltd. and Samsung
Electronics Co., Ltd., for example, and high-luminance LET's having
emission band of ultraviolet region to visible region, manufactured
and sold by Nichia Corporation, Toyoda Gosei Co., Ltd., Lumileds,
Courier, and the like.
[0114] Herein, in a direct back light device using a conventional
common visible light-emitting light source, a direct image of the
light source itself having a high light intensity is viewed, so
that evenness of emission is remarkably impaired. Therefore, it is
necessary to provide a mask for avoiding such direct viewing of the
image or to provide a diffusion material for varying transmittance
just above the light source.
[0115] To the contrary, in a case of the polarized-light-emitting
planar light source obtained by combining the optical element 10
according to the invention and the excitation light source 9, both
of excitation light resulting from the excitation light source 9
and visible light generated by excitation of the luminous body 3
are transmitted within the optical element 10 through scattering by
the minute regions 2 and reflection at the front and rear sides of
the optical element 10. Therefore, as shown in FIG. 6, even if the
excitation light source 9 is supposedly a point light source, the
transmitted excitation light collides with the luminous body 3
anywhere to excite the luminous body 3, thereby visible light being
generated. On the other hand, as mentioned above, when an
excitation light source emitting ultraviolet light or an excitation
light source 9 having an emission band of ultraviolet light to
visible light is used, excitation light itself is not clearly
viewed by eyes, so that the vicinity of the excitation light source
9 is not specifically viewed brightly. Therefore, evenness of
emission on visual light resulting from the
polarized-light-emitting planar light source is relatively good as
far as the luminous body 3 is homogeneously dispersed.
[0116] Moreover, when a material absorbing relatively much light
having the wavelength of excitation light is used as the
translucent resin or the minute regions, the material absorbs the
excitation light and hence emission efficiency tends to be lowered.
Furthermore, when ultraviolet light is used as the excitation
light, deterioration of the material may be invited owing to the
absorption of ultraviolet light. Thus, the use of a material
substantially absorbing no light having the wavelength of
excitation light as a material of the translucent resin or the
minute regions can reduce decrease in emission efficiency and
deterioration of the material as far as possible. Furthermore, in a
case where the excitation light source 9 is an excitation light
source emitting ultraviolet light, both of the translucent resin 1
and the minute regions 2 are both preferably made of materials that
do not substantially absorb ultraviolet light.
[0117] In a case where a material substantially absorbing no light
having the wavelength of excitation light is used is the material
of the translucent resin 1, any of inorganic materials, organic
materials, and mixtures thereof may be employed as such a material
as far as it is a material substantially absorbing no light having
the wavelength of excitation light. Thus, it is possible to select
optional one according to the emission wavelength of the excitation
light source 9. Particularly, in a case where ultraviolet light is
used as excitation light, cyclic polyolefins or polyolefins having
a norbornene structure, and the like may be mentioned, for example.
Moreover, in a case where a material substantially absorbing no
light having the wavelength of excitation light is used as the
material of the minute regions 2, any of inorganic materials,
organic materials, and mixtures thereof, which substantially absorb
no light having the wavelength of excitation light, may be employed
as such a material as far as it satisfies the relation of
refractive index with the translucent resin 1. Particularly, when
an excitation light source emitting ultraviolet light is used as
the excitation light source 9, it is preferred to use crystals of
an inorganic compound having an anisotropic crystal structure, such
as strontium carbonate, lithium niobium trioxide, calcium
carbonate, calcium sulfate dehydrate, potassium phosphate, or
silicon dioxide.
[0118] The optical element 10 according to this embodiment may be
formed with a single layer, or two ore more layers bonded together.
The optical element made through such a multilayer structure or
superimposition can exhibit a scattering effect which is synergized
or enhanced to such a degree higher than an effect resulting from
only increase in thickness. The layers are preferably superimposed
to each other in such a manner as to have the .DELTA.n1 directions
parallel to each other. The number of layers superimposed is two or
more that may be suitably determined.
[0119] The optical element 10 to be superimposed may have
.DELTA.n1, .DELTA.n2 and .DELTA.n3 identical or different in each
layer. Also, the luminous body 3 contained in each optical element
10 may be made of the same or different materials. The layers are
preferably superimposed to each other in such a manner as to have a
parallel relationship in the .DELTA.n1 direction, while
misalignment of the layers due to operational errors or the like is
acceptable to some extent. When the fluctuation of the .DELTA.n1
direction or the like occurs between the layers, these layers are
preferably set with their average directions to have a parallel
relationship with each other.
[0120] A layered structure of the optical element 10 in combination
with an excitation light source, a support member, a light guiding
plate or the like, or a layered structure of plural optical
elements 10 is made by bonding them together via an adhesive layer
or the like so as to make a total reflection interface serve as an
outermost surface of a layered structure. As an adhesive layer, a
hot melt adhesive, pressure sensitive adhesive or any other
suitable type adhesive may be used. In view of suppressing
reflection loss, an adhesive layer having a small refractive index
difference with respect to the optical element 10 is preferably
used. The bonding may be also made by the use of a resin for
forming the light passing resin 1 or the minutes regions 2. As the
above adhesive, for example, an appropriate adhesive including a
transparent adhesive such as acrylic, silicone, polyester,
polyurethane, polyether or rubber adhesive can be used without
particular limitation, while it is preferable to use an adhesive
that does not require application of high temperature for curing or
drying, or does not require a long time for curing or drying, in
view of prevention of changes in optical characteristics or the
like. Also, a resin that is unlikely to cause a so-called
delamination phenomenon such as layer-lifting or layer-peeling
under heating or humidification conditions is preferable.
[0121] Therefore, as the adhesive, it is preferable to use an
acrylic pressure sensitive adhesive containing an acrylic polymer
as the base polymer having a weight-average molecular weight of
100,000 or more, resulting from copolymerization of an alkyl ester
of (meth)acrylic acid having an alkyl group having 20 or less
carbon atoms, such as a methyl group, an ethyl group or a butyl
group, with an acrylic monomer comprising a modifying component
such as (meth)acrylic acid of hydroxyethyl(meth)acrylate, in such a
combination as to have a glass transition temperature of 0.degree.
C. or lower. The acrylic pressure sensitive adhesive has an
advantage in transparency, weather resistance, heat resistance and
the like.
[0122] The adhesive layer may be attached to the optical element 10
by any method appropriate to each case. Specifically, there may be
mentioned a method of melting or dispersing adhesive ingredients
into a solvent made of any one of toluene, ethyl acetate and the
like or mixture thereof to prepare an adhesive solution of about 10
to 40% by weight and directly applying the adhesive solution on the
optical element 10 by a suitable spreading method such as a
flow-casting or coating method, or a method of forming an adhesive
layer on a separator following the above steps and transferring the
adhesive layer onto the optical element 10. The adhesive layer to
be attached can be formed in layered structure having different
compositions or types.
[0123] The thickness of the adhesive layer is appropriately set
according to adhesive power or the like, while it is generally set
in the range of 1 to 500 .mu.m. It is also possible to
appropriately mix an additive such as a natural resin, a synthetic
resin, glass fibers, glass beads, a filler made of metal powder or
other inorganic powder, a pigment, a color agent, or an antioxidant
in the adhesive layer according to needs and circumstances.
[0124] In the example illustrated in FIG. 4, a translucent sheet 4
having an excellent smoothness is bonded on the optical element 10
via an adhesive layer 8 as described above, in which a smooth
surface (an upper side) of the translucent sheet 4 bonded serves as
a total reflecting interface.
[0125] The optical element 10 is preferably structured so as to
entirely or partially have a phase difference in view of the
necessity to appropriately eliminate a polarized state during light
transmits through the optical element 10. Basically, the slow axis
(the axis in the .DELTA.n1 direction) of the optical element 10 has
an orthogonal relationship with the polarization axis (plane of
vibration) of the linearly polarized light, along which light is
hard to be scattered, and therefore polarization conversion due to
phase Clarence is hard to occur. However, it is assumed that slight
scattering causes changes in apparent angle and hence causes
polarization conversion.
[0126] From the point of view of causing the polarization
conversion, the optical element 10 preferably has a phase
difference between in-plane directions of 5 nm or greater in
general, while this phase difference may be varied according to the
thickness of the optical element 10. A preferable upper limit of
the phase difference between in-plane directions of the optical
element is not categorically determined. This phase difference can
be given by employing an appropriate method, such as a method of
incorporating birefringent fine particles in the optical element 10
or a method of attaching the same on the optical element 10, a
method of giving the birefringent characteristics to the
translucent resin 1, a method of employing these methods in
combination, or a method of forming birefringent films into an
integral laminate structure.
[0127] In order to allow the optical element 10 to efficiently emit
polarized light through one of the front and rear sides thereof in
the polarized-light-emitting planar light source, to which the
optical element 10 according to this embodiment is applied, a
reflection layer 5 is preferably located as illustrated in FIG. 5.
In the example as illustrated in FIG. 5, the reflection layer 5 is
located on the rear aide (lower side) of the optical element 10, so
that light emitted through the rear side of the optical element 10
is reversed via the reflection layer 5 without change in a
polarized state and the thus emitted light is concentrated on the
spice of the optical element 10. Whereby, the luminance of the
optical element 10 can be enhanced.
[0128] The reflection layer 5 preferably has a mirror surface in
order to sustain the polarized state. For this purpose, it is
preferable to form the reflection layer 5 having a reflection
surface made of a metal or dielectric multilayer film. As the
metal, aluminum, silver, chrome, gold, copper, in, zinc, indium,
palladium or platinum, or their alloy can be appropriately
used.
[0129] The reflection layer 5 may be directly brought into tight
contact with the optical element 10 as an attached layer of a metal
thin film by vapor deposition, but is hard to produce perfect
reflection and hence causes slight absorption by the reflection
layer 5. Accordingly, in view of the fact that the total reflection
of the light transmitting in the optical element 10 is repeated,
the direct tight contact of the reflection layer 5 to the optical
element 10 may cause absorption lose. In order to prevent this
absorption loss, it is preferable to only overlay the reflection
layer 5 on the optical element 10 (i.e., allowing air to be
interposed between).
[0130] Accordingly, as the reflection layer 5, it is preferable to
use a reflection plate having a substrate with a metal thin film
attached thereon by sputtering or vapor deposition, or a plate-like
member such as metal foil or rolled metal sheet. As the substrate,
it is possible to appropriately use a glass plate, resin sheet or
the like. Particularly, the reflection layer 5 is preferably formed
by vapor deposition of silver, aluminum or the like on a resin
sheet in view of reflectivity, hue, handling property or the
like.
[0131] On the other hand, as the reflection layer 5 made of a
dielectric multilayer film, a film disclosed in JP-T-10-511322 or
the like can be appropriately used.
[0132] In addition to the arrangement of locating the reflection
layer 5 on the rear side of the optical element 10 as illustrated
in FIG. 4, it is possible to locate the reflection layer 5
anywhere, for example, on the front side or lateral side of the
optical element 10, or in a case of the arrangement with a light
guide plate, on the front, rear or lateral side thereon or any
other place appropriate to each case.
[0133] As illustrated in FIG. 4, in the polarized-light-emitting
planar light source to which the optical element 10 according to
this embodiment is applied, a polarization-maintaining lens sheet
7, a light diffusion layer 6 or the like may be located on a
light-retrieving side (upper side) of the optical element 10. Also,
it is possible to appropriately locate a wavelength cut filter (not
shown) or a retardation film (not shown).
[0134] The lens sheet 7 is provided so as to control optical path
of the light (linearly polarized light) emitted from the optical
element 10, while maintaining its polarization, so as to improve
the directivity toward the front side, which is advantageous in
visual recognition characteristics, and so as to allow the emitted
light having scattering characteristics to have an intensity peak
on the front side.
[0135] As the lens sheet 7, an appropriate type of lens sheet may
be used without particular limitation, which is capable of
controlling the optical path of the scattered light entered through
one of the opposite sides (rear side) of the optical element 10 and
efficiently emitting the lift through the other side (front side)
in a direction orthogonal to the sheet surface (in the front
direction). Therefore, except for the polarization-maintaining
characteristics, it is possible to use any lens sheet having a
varying lens form, as disclosed in JP-A-5-169015, which is used in
a conventional, so-called sidelight-type light guide plate.
[0136] As the lens sheet 7, it is preferable to use a lens sheet
having an excellent transmittivity, for example, with a total
transmittance of the light being preferably 80% or higher, more
preferably 85% or higher and particularly preferably 90% or higher,
and with a transmittance of the light leaked as a result of
eliminating the polarization being preferably 5% or lower, more
preferably 2% or lower and particularly preferably 1% or lower in a
case where the lens is set in a cross-Nicol position, as well as
enabling emission of light still possessing the polarization
characteristics.
[0137] In general, the elimination of the polarization is caused by
birefringence, multiple scattering or the like, and therefore the
lens sheet 7 exhibiting the polarization-maintaining
characteristics can be achieved by reducing the birefringence, or
reducing an average number of reflections (scatterings) of light
transmitting in the lens. Specifically, it is possible to prepare
the lens sheet 7 with the polarization-maintaining characteristics
by the use of one or more of resins having small birefringence
characteristics (resins having an excellent optically isotropic
characteristics), such as cellulose triacetate resin, polymethyl
methacrylate, polycarbonate, norbornene resin or the like, which
are exemplified in the above as a polymer used for the optical
element 10.
[0138] The lens sheet 7 may be of various lens forms such as a lens
form with a large number of lens regions (particularly minute lens
regions) of a convex lens type or a refractive index distribution
type (GI type), made of a transparent resin substrate, which may
contain a resin having a different refractive index, and
photopolymer placed on or inside of the resin substrate so that a
refractive index is controlled through the photopolymer; a lens
form with a lens region made of a transparent resin substrate
formed with a large number of through-holes in which a polymer
having a different refractive index is filled; or a lens form with
a large number of spherical lenses arranged in a single layer and
fixed within a thin film. However, in view of the optical path
control by setting different refractive indexes, it is preferable
to use a lens sheet wherein a lens configuration 71 having an
irregular surface structure is provided on the surface of the lens
sheet 7.
[0139] The irregular surface structure, which forms the lens
configuration 71, may be varied, as far as it can control the path
of light, which has been transmitted through the lens sheet 7, so
as to concentrate the transmitted light towards the front side. For
example, there may be mentioned an irregular surface structure
having a large number of linear grooves having triangular cross
section and protrusions alternately aligned parallel or arranged in
lattice pattern, or an irregular surface structure having a large
number of minute protrusions each having a bottom of a
triangular-pyramid, quadrangular-pyramid, or polygonal-pyramid
vertex, which are arranged in dot patterns. The irregular surface
structure in a linear or dot pattern may be a spherical lens,
aspheric leas, half-round lens or the like.
[0140] The lens sheet 7 having an irregular surface structure in a
linear or dot pattern can be formed by an appropriate method such
as a method of filling a resin solution or resin-forming monomer
into a mold having a molding surface conformed to create a
predetermined irregular structure, optionally subjecting the filled
solution or monomer to polymerization according to needs and
circumstances and then transferring the molded irregular structure
onto a target surface, or a method of heating a resin sheet and
pressing the same into the aforesaid mold to transfer the irregular
surface structure onto a target surface. The lens sheet 7 may be of
a layered structure with two or more resin layers of the same or
different types, such as a lens sheet made of a substrate sheet to
which a lens form is applied.
[0141] One or more layers of the lens sheet 7 may be located on the
light-emitting side of the optical element 10. In a case where two
or more layers of the lens sheets 7 are located, they may be of the
same type as each other or different types from each other, while
it is preferable to exhibit the polarization-maintaining
characteristics throughout the entirety thereof. In a case where
the lens sheet 7 is located in proximity with the optical element
10, the lens sheet 7 is preferably located with a clearance to the
optical element 10, that is, to have an air layer interposed
therebetween, in the same manner as in the case of the reflection
layer 5. It is preferable that the clearance is sufficiently
greater than a wavelength of the incident light.
[0142] In a case where the lens for, of the lens sheet 7 has an
irregular surface structure in linear pattern, it is preferable to
locate the lens sheet 7 so as to allow the linearly aligned members
of the irregular surface structure to be oriented parallel or
orthogonal to the optical axis direction of the optical element 10
(a direction of the plane of vibration of the emitted polarized
light) in order to provide appropriate control of the optical path
towards the front side. When two or more layers of the lens sheets
7 are located, it is preferable to locate them to have the aligned
directions of the linearly aligned members thereof crossing each
other in view of efficient optical path control.
[0143] The light diffusion layer 6 serves to, for example, equalize
the light emission by scattering light emitted from the optical
element 10 while maintaining the polarization thereof, or limit the
irregular surface structure of the lens sheet 7 from being
visualized so as to improve the visual recognition characteristics
and the like.
[0144] As the light diffusion layer 6, it is preferable to use one
having excellent transmittivity of light and
polarization-maintaining characteristics for the emitted light as
in the case of the lens sheet 7. Therefore, the light diffusion
layer 6 is preferably formed by the use of a resin having small
birefringence characteristics such as those exemplified for the
lens sheet 7. For example, it is possible to form the light
diffusion layer 6 having the polarization-maintaining
characteristics by dispersedly distributing transparent particles
in the resin, or providing a surface with a resin layer having a
minute irregular surface structure.
[0145] As transparent particles to be dispersedly distributed in
the above resin, there may be mentioned inorganic fine particles
made of silica, glass, alumina, titania, zironia, tin oxide, indium
oxide, cadmium oxide, antimony oxide or the like that may have
electric conductivity, or organic fine particles made of a
crosslinked or uncrosslinked polymer such as an acrylic polymer,
polyacrylonitrile, a polyester, an epoxy resin, a melamine resin, a
urethane resin, polycarbonate, polystyrene or a silicone resin,
benzoguanamine, melamine, benzoguanamine condensate, or
benzoguanamine-formaldehyde condensate.
[0146] One or more materials are used to make the transparent
particles, and the particle size is preferably 1 to 20 .mu.m in
diameter in view of light diffusing capability, equal diffusion
characteristics or the like. While the particle shape is optionally
determined, a (true) spherical shape, its secondary aggregate or
the like is generally used. Particularly, it is preferable to use
transparent particles having a refractive index ratio of 0.9 to 1.1
to the resin in view of the polarization-maintaining
characteristics.
[0147] The light diffusion layer 6, which contains the
aforementioned transparent particles, can be formed by an
appropriate known method, such as a method of incorporating
transparent particles into a molten resin solution and extruding it
into a sheet or the like, a method of blending transparent
particles into a resin solution or monomer and then casting the
solution into a sheet or the like, and optionally subjecting it to
polymerization according to needs and circumstances, or a method of
applying a resin solution containing transparent particles on a
predetermined surface or a substrate film having the
polarization-maintaining characteristics.
[0148] On the other hand, the light diffusion layer 6 having minute
irregular surface structures can be formed by an appropriate
method, for example, a method of roughening the surface of a sheet
made of a resin by buffing such as sandblasting or embossing
finish, or a method of forming a layer of a translucent material on
the surface of the resin sheet so as to provide protrusions
thereon. However, it is not preferable to employ a method of
forming protrusions having a large refractive index difference to
the resin, such as air bubbles or titanium oxide fine particles
because a minute irregular surface structure formed by this method
tends to eliminate the polarization.
[0149] The minute irregular surface of the light diffusion layer 6
preferably has a surface roughness higher than the wavelength of
the incident light but not higher than 100 .mu.m in view of light
diffusing characteristics, its equal diffusion characteristics or
the like, and preferably have an irregular pattern with no
periodicity.
[0150] For forming the light diffusion layer 6 of the above types
that contains transparent particles or has a minute irregular
surface, it is preferable to limit increase in phase difference due
to photoelasticity or orientation, particularly in a base layer
made of the aforementioned resin in view of the
polarization-maintaining characteristics.
[0151] The light diffusion layer 6 may be arranged in the form of
an independent layer having such as a plate-like shape, or a
dependent layer internally formed with the lens sheet 7 in tight
contact with each other. When the light diffusion layer 6 is
located adjacent to the optical element 10, it is preferable to
locate them to have a clearance therebetween in the same manner as
in the case of the lens sheet 7. When two or more layers of the
light diffusion layers 6 are provided, they may be of the same type
as each other or different types from each other, while it is
preferable for them to exhibit the polarization-maintaining
characteristics throughout the entirety thereof.
[0152] The wavelength cut filter as mentioned above is used for the
purpose of preventing direct light from the excitation light source
9 from entering a liquid crystal display element or the like, which
is illuminated by the polarized-light emitting-planar light soured
according to this embodiment. Particularly, in a case where
excitation light is ultraviolet light, a wavelength cut filter is
preferably used in order to prevent deterioration of liquid
crystal, polarizing plate or the like due ultraviolet light. The
wavelength cut filter may also be used for the purpose of
eliminating visible light rays of unnecessary wavelength.
[0153] As the wavelength cut filter, there may be mentioned a film
that is made by dispersing a material, which absorbs a target
wavelength (e.g., an UV absorber such as an salicylate ester
compound, a benzophenol compound, a benzotriazole compound, a
cyanoacrylate compound, or a nickel complex salt compound), in a
resin capable of allowing visible light to pass therethrough or
applying the material on the resin, a film made of a translucent
film with a cholesteric liquid crystal layer formed thereon, a film
that reflects light of a target wavelength through the reflection
of a dielectric multilayer film, or the like. It is also possible
to incorporate an UV absorber or the like in the optical element 10
or any other optical member, enabling the optical element 10 or any
other optical member itself to serve to cut wavelength.
[0154] The retardation film as mentioned above is used for the
purpose of converting linearly polarized light emitted from the
optical element 10 into light in a given polarized state. For
example, it is possible to convert linearly polarized light into
circular polarized light by the a engagement that a quarter-wave
plate as a retardation film is located to have a slow axis oriented
45.degree. to the linearly polarized light emitted, or rotate the
polarization axis of the linearly polarized light emitted from the
optical element 10 by using a half wave plate.
[0155] As the retardation film, there may be mentioned a film
comprising a polymer film, which is generally used for compensating
liquid crystal cells, a film comprising a translucent film having
an oriented liquid crystal polymer or the like attached thereon, or
the like.
[0156] Each of the lens sheet 7, the light diffusion layer 6, the
wavelength cut filter and the like described in the above may be
used as a separate layer, or some or an of them may make up a
single film in laminate structure. Also, they can be tightly bonded
via an adhesive layer or the like to a liquid crystal display
element to be located thereon. However, for the lens sheet 7 having
an irregular surface structure or the light diffusion layer 6
having a minute irregular surface structure mentioned above, it is
preferable to locate them with a distance to the liquid crystal
display element.
[0157] It is also preferable to locate each of the lens sheet 7,
the light diffusion layer 6, the wavelength cut filter and the like
with a distance to the optical element 10 so as to prevent the
control of the condition of the critical angle within the optical
element 10 in view of retrieving polarized light in an efficient
manner.
[0158] The optical element 10 according to this embodiment and the
polarized-light-emitting planar light source, to which the optical
element 10 is applied, is capable of allowing light, which results
from excitation by incident light from the excitation light source
9, to be emitted from the optical element 10 in the form of
linearly polarized light, and also capable of controlling the
polarization direction (the plane of vibration). Therefore, they
are suitably applicable in various devices or to various fields,
such as a liquid crystal display that utilizes linearly polarized
light.
EXAMPLES
[0159] Examples and comparative examples will be provided in order
to further distinguish the features of the present invention. The
following will describe modes for carrying out the invention in
detail with reference to Examples but the invention is not limited
to these Examples.
Example 1
(1) Material for Preparing Optical Element
[0160] POVAL PVA 124 (degree of polymerization: 2400), a polyvinyl
alcohol manufactured by Kuraray Co., Ltd., a liquid crystal monomer
UCL008 manufactured by Dainippon Ink and Chemicals, Incorporated,
and a dispersion liquid (corresponding to 20% by weight) of ZnS
nanoparticles (particle size: 2 to 4 nm,) manufactured by Sumitomo
Osaka Cement Co., Ltd. were used as a translucent resin, a material
for preparing minute regions, and a luminous body, respectively.
Furthermore, a fluorine-based leveling agent, Megafac manufactured
by Danippon Ink and Chemicals, Incorporated was used as a leveling
agent.
(2) Preparation of Polyvinyl Alcohol Solution
[0161] The above polyvinyl alcohol was dissolved in hot water to
prepare a 13% aqueous solution. To such an aqueous polyvinyl
alcohol solution (aqueous PVA solution) was added glycerin in an
amount corresponding to 15% by weight based on the solid matter. On
the other hand, 2.9 g of the above liquid crystal monomer, 0.014 g
of the above leveling agent, and 2.9 g of the above luminous body
(solid matter) were mixed with each other and the whole was heated
and stirred until an isotropic phase was formed. Then, after they
became homogeneous, 450 g of the above aqueous PVA solution heated
at 90.degree. C. was added thereto and mixed. The mixing was
conducted at 6000 rpm for 20 minutes using a homomixer. The
resulting mixture was allowed to stand for 24 hours in a warm state
kept at 35.degree. C. to obtain a bubble-free homogeneous polyvinyl
alcohol solution.
(3) Film Formation
[0162] The above polyvinyl alcohol solution was applied in a wet
thickness of 1 mm by means of an applicator and subjected to drying
conditions of 110.degree. C..times.20 minutes and annealing
conditions of 140.degree. C..times.4 minutes to obtain a dried base
material.
(4) Stretching
[0163] The above base material was stretched to 400% extension in
an aqueous boric acid solution (4% by weight, 60.degree. C.),
thereby an optical element being prepared.
[0164] With regard to the above optical element, refractive index
difference .DELTA.n1 was 0.15 and each of .DELTA.n2 and .DELTA.n3
was 0.01. At the measurement of the refractive indices, refractive
index was measured by means of an Abbe refractometer on an optical
element wherein polyvinyl alcohol was solely subjected to
stretching under the same conditions as above or an optical element
wherein the above liquid crystal monomer was applied on an
orientation film, then oriented and fixed. Then, differences
therebetween were calculated as .DELTA.n1, .DELTA.n2, and
.DELTA.n3. The luminous body was present mainly in polyvinyl
alcohol in a dispersed state. Moreover, when average length of the
minute region (liquid crystal monomer) was measured by coloration
based on retardation by polarized microscopic observation, length
in a long-axis direction was about 5 .mu.m and length in a
short-axis direction was about 1.5 .mu.m.
Example 2
[0165] An optical element was prepared in accordance with Example 1
except that the polyvinyl alcohol solution was applied in a wet
thickness of 2 mm and the dried base material was stretched to 500%
extension.
Example 3
[0166] A film was formed by casting using a 25% by weight toluene
solution containing 94 parts (parts by weight, the same shall apply
hereinafter) of a norbornene resin (ARTON manufactured by JSR
Corporation, glass transition temperature: 182.degree. C.), 5 parts
of strontium carbonate as a material of preparing minute regions,
and 1 part of ZnS nanoparticles (manufactured by Sumitomo Osaka
Cement Co., Ltd., excitation wavelength: 345 nm, emission
wavelength: 580 nm) dissolved therein. Then, the film was heated
from 50.degree. C. to 120.degree. C. at a constant gradient and
dried for 1 to 2 hours. Thereafter, the film was stretched at
170.degree. C. to 200%, extension to prepare an optical element
having a thickness of 80 .mu.m.
Example 4
[0167] An optical element was prepared in accordance with Example 3
except that silicon dioxide was used instead of strontium
carbonate.
[0168] In this connection, Table 1 shows light absorption
wavelengths of individual materials used for preparation of the
optical elements according Examples 3 and 4. In Table 1, numerals
described in the columns of the translucent resin and the minute
regions mean light absorption wavelength bands. Moreover, numerals
described in the column of the luminous body mean excitation
wavelengths. Furthermore, numerals described in the column of the
excitation light source mean central wavelengths of emitted light.
TABLE-US-00001 TABLE 1 Excitation light Translucent resin Minute
regions Luminous body source Example 3 Norbornene resin Strontium
carbonate ZnS nanoparticles Ultraviolet LED less than 300 (nm) less
than 300 (nm) 345 nm 365 nm Example 4 Norbornene resin Silicone
oxide ZnS nanoparticles Ultraviolet LED less than 300 (nm) less
than 200 (nm) 345 nm 365 nm Referential Norbornene resin Liquid
crystal polymer ZnS nanoparticles Ultraviolet LED Example less than
300 (nm) less than 450 nm 345 nm 365 nm
Reference Example
[0169] An optical element was prepared in accordance with Example 3
except that a material that absorbed relatively much light of
excitation light wavelength (specifically, the liquid crystal
polymer represented by the following chemical formula glass
transition temperature of 70.degree. C., nematic liquid
crystallization temperature of 190.degree. C.) was used as a
material for preparing minute regions instead of strontium
carbonate used in Example 3. In this connection, Table 1 shows
light absorption wavelengths of individual materials used for
preparing the optical element according to the present Reference
Example. ##STR2##
Comparative Example 1
[0170] An optical element was prepared in accordance with Example 1
except that there was used, as a luminous body, one wherein ZnS
manufactured by Wako Pure Chemical Industries, Ltd. was pulverized
in a homogenizer to form particles having an average particle size
of 1 .mu.m and a maximum particle size of 10 .mu.m.
Comparative Example 2
[0171] A film having a thickness of 100 .mu.m was prepared by
casting using a 20% by weight dichloromethane solution containing
950 parts (parts by weight the same shall apply hereinafter) of a
norbornene resin (ARION manufactured by JSR Corporation, glass
transition temperature: 182.degree. C.), 50 parts of a liquid
crystal polymer represented by the following chemical formula
(glass transition temperature: 80.degree. C., temperature for
nematic liquid crystal: 100.degree. C. to 290.degree. C.) and 2
parts of 3-(2-benzothiazolyl)-1-diethylaminocoumarin (coumarin 540)
dissolved therein. The film was stretched at 180.degree. C. to 300%
extension and then rapidly cooled, thereby an optical element being
prepared. ##STR3##
[0172] The optical element thus formed was constituted by a
transparent film made of a norbornene resin and a liquid crystal
polymer dispersed therein as domains of about the same shape
elongated in the stretch direction and had a refractive index
difference .DELTA.n1 of 0.23 and refractive index differences
.DELTA.n2 and .DELTA.n3 of 0.029. At the measurement of the
refractive indices, refractive index was measured by means of an
Abbe refractometer on an optical element wherein the norbornene
resin was solely subjected to stretching under the same conditions
as above or an optical element wherein the above liquid crystal
monomer was solely applied on an orientation film, then oriented
and fixed. The differences between the measured refractive indexes
were respectively calculated as .DELTA.n1, .DELTA.n2 and .DELTA.n3.
Coumarin was present in a molten state in the norbornene resin. The
average particle size of minute regions (domains of the liquid
crystal polymer) was measured by coloration through polarizing
microscopic observation on the basis of the phase difference. As a
result, it has been found that the length in the .DELTA.n1
direction was about 5 .mu.m.
[0173] After bonding the optical element of the Example 1 to a
glass plate (thickness: 3 mm) by using an acrylic adhesive, a
silver-deposited mirror-finished reflective sheet, which was
prepared by vapor deposition of silver on a polyethylene
terephthalate sheet, was located on the side opposite to side on
which the glass plate was bonded, to prepare a multilayer member,
and a black-light cold cathode fluorescent lamp was fixed on any
one of the opposite sides of the multilayer member by a lamp
reflector of a mirror-finished reflective sheet. Thus, a
polarized-light-emitting planar light source was formed.
Evaluation
[0174] With regard to the optical elements of Examples 1 and 2 and
Comparative Example 2, breakage did not occur and defective
appearance was not observed during the preparation. To the
contrary, with regard to the optical element of Comparative Example
1, there was observed defective appearance that large particles of
the luminous body protruded from the surface and fine concavity and
convexity are formed at film formation. Furthermore, cracks were
formed start from large particles of the luminous body and then the
film was broken at stretching.
[0175] Using a point light source, an ultraviolet emission LED
(NSHU 590A) manufactured by Nichia Corporation as an excitation
light source for allowing excitation light to enter the optical
elements of Examples 1 and 2 and Comparative Example 1, ultraviolet
light was emitted at 15 mA and allowed to enter each optical
element. On the optical elements of Examples 1 and 2, the output
intensities of the respective components of linearly polarized
light in the .DELTA.n1 direction and the .DELTA.n2 direction of
emitted light were measured using a commercially available
polarizer (a 99.99 degree of polarization). As a result, it was
found that a linearly polarized light was emitted uniformly almost
all over the surface of the optical element in a ratio of 4:1 in
the optical element of the Example 1 and 6:1 in the optical element
of the Example 2. To the contrary, on the optical element of
Comparative Example 1, strong light scattering was observed at the
luminous body and hence a linearly polarized light in a ratio of
about 1.5:1 was only obtained.
[0176] On the other band, green luminescence having a center
wavelength of 505 nm was observed upon irradiation of the optical
element of the optical element of Comparative Example 2 with light
emitted from a backlight fluorescent lamp (center wavelength of 360
nm) as excitation light. The output intensities of the respective
components of linearly polarized light in the .DELTA.n1 direction
and the .DELTA.n2 direction of emitted light were measured by using
a commercially available polarizer (a 99.99 degree of
polarization). Then, it was found that a linearly polarize light
was emitted in a ratio of 6:1.
[0177] Also, it was found that, in the polarized-light-emitting
planar light source of Comparative Example 2, linearly polarized
light of the optical element in the .DELTA.n1 direction was emitted
in plane. However, with regard to the polarized-light-emitting
planar light source of Comparative Example 2, coumarin was
deteriorated in thermal reliability test after the treatment of
90.degree..times.24 hours and luminance of emitted light was
remarkably lowered.
[0178] In addition, on the optical elements of Examples 3 and 5,
color reproduction by eyes as well as material deterioration and
emission efficiency by ultraviolet light absorption were
evaluated.
(1) Color Reproduction (Visual Recognition Characteristics)
[0179] When the optical elements obtained in Examples 3 and 4 were
irradiated with light from ultraviolet LED having a sharp peak
having a center wavelength of 365 nm as excitation light, red light
having a peak wavelength of 580 nm was observed but the other
colors were not visually observed. On the other hand, purplish
color mixed in red light was visually observed upon irradiation
with light emitted from a black lamp (center wavelength of 370 nm)
having a gentle-slope peak containing wavelengths in a visible band
of 350 nm to 400 nm as excitation light. This may be because part
of visible light of 400 nm or shorter contained in light from the
black lamp as the excitation light source is transmitted through
the optical element and is visually observed. Therefore, it was
found that it is preferred to use an excitation light source
emitting ultraviolet light containing no waveless within a visible
band.
(2) Emission Efficiency
[0180] When light emission of the optical elements according to
Examples 3 and 4 and Reference Example was investigated using the
above ultraviolet LED, it was confirmed that the optical elements
according to Examples 3 and 4 exhibited emission efficiency about
40% higher than that of the optical element according to Refereuce
Example.
(3) Material Deterioration by Ultraviolet Absorption
[0181] After the optical elements according to Examples 3 and 4 and
Reference Examples were subjected to an irradiation test with
ultraviolet light (irradiation with ultraviolet light having a
radiation intensity of 500 W/m.sup.2 for 3 days), the output
intensities of the respective components of linearly polarized
light in the .DELTA.n1 direction and the .DELTA.n2 direction of
emitted light were measured using an ultraviolet LED as an
excitation light source. As a result, linearly polarized light was
emitted in a ratio of 5:1 in the optical elements of the Examples 3
and 4 but in a ratio of about 1:1 in the optical element of
Reference Example. This may be because the liquid crystal as minute
regions is deteriorated by the irradiation test with ultraviolet
light and hence the anisotropy of the liquid crystals is lost.
[0182] While the present invention has been described in detail and
with reference to specific embodiments thereof it, will be apparent
to one skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope
thereof.
[0183] The present application is based on Japanese Patent
Application No. No. 2004-88122 filed on Sep. 30, 2004 and Japanese
Patent Application No. 2005-122721 filed on Apr. 20, 2005, and the
contents thereof are incorporated herein by reference.
INDUSTRIAL APPLICABILITY
[0184] According to the present invention, there can be provided an
optical element that is capable of allowing light, which results
from excitation by incident light to be emitted through at least
one of the front and rear sides of the optical element in the form
of linearly polarized light having a sufficient degree of
polarization and that is prepared without occurrence of defective
appearance and is capable of easily enhancing the luminance of
emitted light, as well as a polarized-light-emitting planar light
source using the optical element and a display device using the
same.
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